The present invention relates to an ink jet recording apparatus, that is, an image forming apparatus which forms images on recording medium by forming numerous ink dots on the recording medium by ejecting onto the recording medium, and a method for preventing an ink jet recording apparatus from forming defective images, in particular, a method for compensating for the specific unwanted properties of the recording heads of an ink jet recording apparatus, more specifically, compensating for the recording head nozzles which do not eject ink straight, recording head nozzles which fail to eject ink, etc.
In recent years, various information processing devices such as copying machines, wordprocessors, computers, etc., and also, communication devices, have come into general use. Consequently, digital recording devices employing a single or plurality of ink jet recording heads have rapidly come into general use, as one of the image forming (recording) apparatuses for the information processing devices and communication devices. Also in recent years, information processing devices and communication devices have been colorized, being therefore drastically improved in terms of visual information. Consequently, the demand for recording apparatuses higher in image quality, and for the colorization of recording apparatuses have been increasing.
In order to reduce picture element size, increase recording speed, etc., these new types of recording apparatuses employ a recording head (which hereinafter may be referred to as multi-head) comprising a plurality of integrally arranged recording elements. Each recording element comprises an ink ejection orifice and a liquid path thereto. Thus, a multi-head comprises a plurality of ink ejection orifices and liquid paths which are integrally arranged at a high density. Generally, color image forming apparatuses comprise a plurality of the above described multi-heads which correspond one for one to cyan, magenta, yellow and black inks, for example, to effect various colors.
The primary concerns in the technological field of this ink jet recording apparatus are how to improve recording speed, how to reduce recording cost, and how to improve image quality, while maintaining the above described structural arrangement. As one of the means for improving recording speed, a method in which the length of a multi-head is approximately matched with the width of recording medium, so that the multi-head has to pass the recording medium only once, has been realized.
However, this method has the following weakness. That is, for example, in order to enable a page printer to accommodate an A4 recording paper positioned so that the shorter edges become parallel to the direction in which the recording paper is conveyed through the printer, the multi-head of this page printer must be no less than approximately 30 cm in length, requiring no less than 7000 nozzles, provided that the resolution is 6000 psi. From the standpoint of yield, it is extremely difficult to produce a large number of flawless multi-heads having this many nozzles. Further, because of the shear number of nozzles, there is no guarantee that all nozzles are equal in performance. Moreover, there is a substantial possibility that some nozzles will stop ejecting ink while in use.
The head shading technologies for compensating for the nonuniformity among the nozzles, in terms of the amount by which recording liquid is ejected therefrom, as well as in terms of the deviation of the liquid droplets in terms of the target landing points have been attracting attention. Further, the technologies for compensating for the failing nozzles so that even a multi-head, all the nozzles of which are not flawless, can be employed, have also been attracting attention.
According to the most commonly used head shading method, a predetermined pattern (for example, pattern in which dots are arranged in zigzags, duty ratio of which in 50%, which hereinafter may be referred to as a zigzag pattern) is printed, and the printed pattern is measured in density while establishing the positional relationship between a specific point of the printed pattern and a specific nozzle. Then, the performance of each nozzle in terms of density is calculated from the measurement, and the image formation data are modified according to the calculated performance of each nozzle.
For example, if a given nozzle is smaller in the amount of the recording liquid ejected therefrom compared to those from the other nozzles, and therefore, the area of an image corresponding to this nozzle is lower in density, the image formation data are modified so that the gradation value for the area corresponding to this nozzle is increased in order to output images uniform in density.
Further, the technologies for compensating for abnormal nozzles other than the above described nozzles which are smaller in the amount of the liquid they eject, nozzles greater in the ejection direction deviation, nozzles unable to eject, etc., have been proposed. An abnormal nozzle is treated as a nozzle unable to eject, even if it is capable of ejecting. Therefore, it may be referred to as a xe2x80x9cnon-ejection nozzlexe2x80x9d. Thus, hereinafter, the technologies for recording images while compensating for non-ejection nozzles will be referred to simply as xe2x80x9cnon-ejection compensation technologyxe2x80x9d.
The flow resistance of an ink path is affected by production errors, sedimentation of foreign substances therein, etc. Therefore, it is more likely than not that the nozzles of a long head, such as the aforementioned long head having approximately 7000 nozzles, become different in refill properties during their service lives.
As long as the relationship between the recording conditions and refill properties of the recording head is such that the amount of the liquid to be ejected from each nozzle per unit of time is smaller than the amount by which each nozzle is refilled per unit of time, there is no problem. However, as some nozzles deteriorates in refill properties, that is, as the amount by which these nozzles are refilled per unit of time becomes smaller than the amount by which liquid is ejected therefrom per unit of time, the amount by which liquid is ejected therefrom reduces, or in the worst case, ink is not ejected therefrom. This condition will be referred to as xe2x80x9cinsufficient refill non-ejectionxe2x80x9d. It is rather difficult to determine the above described refill properties of each nozzle, based on the aforementioned nozzle check pattern for checking whether or not a given nozzle is literally a non-ejection nozzle, for the following reason. That is, this nozzle check pattern is not likely to allow a given nozzle to fail to eject as long as the given nozzle is the only nozzle among the nozzles in its adjacencies which is caused to eject ink. In other words, even if the flow resistance is increased by a piece of sedimentation 72 as schematically shown in FIG. 7, ink is supplied to the adjacencies of a heater 71 from the adjacencies of the sedimentation 72. Thus, when a given nozzle is allowed to eject ink while preventing the nozzles in the adjacencies thereof from ejecting, as is when the aforementioned nozzle check pattern is printed, there is not likely to be a delay in ink supply Therefore, the nozzle ejects ink like a normal nozzle, making it difficult to determine whether or not this nozzle is normal. In other words, even if a given nozzle is recognized as a non-ejection nozzle while ink is continuously ejected from this nozzle during an image recording operation, if sometimes fails to be recognized as a non-ejection nozzle while the nozzle check pattern is printed, that is, while ink is ejected at a relatively low duty ratio.
This creates the following problems. That is, when the presence of nozzles inferior in refill properties, even though the head shading or non-ejection compensation is effective when printing the low duty areas of an image, they are not effective enough when printing the high duty areas of the image. This results from the following reason: as compensation is made by carrying out the head shading process on the nozzle corresponding to the areas, the density of which has reduced due to the insufficient refill performance (insufficient refill non-ejection), that is, as the number of recording dots corresponding to the areas is increased by the head shading process to increase the density thereof, the head shading process itself requires better refilling performance, having therefore adverse effects.
Thus, the primary object of the present invention is to make it possible to compensate for the increases in the flow resistance of an ink path for some reasons.
The present invention was made to accomplish the above described object, and according to one of the characteristic aspect of the present invention, a method for compensating for the abnormal nozzles of the recording head of an ink jet recording apparatus, that is, a recording apparatus which employs a recording head comprising a plurality of nozzles for ejecting ink, and which forms an image on recording medium by ejecting ink onto the recording medium, comprises; an image outputting process for outputting two or more images of a pattern, uniform in gradation, for measuring the recording properties of the recording head; a measuring process for measuring the density distribution of the outputted images of the pattern; a calculating process for calculating the data for compensating for each of the plurality of the nozzles, based on the results of the measurements of each of the images of the pattern; a sorting process for sorting the plurality of the nozzles into a plurality of groups different in properties, by making comparison among two or more sets of data corresponding to the aforementioned two or more images of the pattern; and a compensating process for making appropriate compensation for each group of nozzles, based on the properties which characterize the group, wherein the two or more images of the pattern outputted in the image outputting process are made different in the recording frequency at which the recording head is driven, and wherein the compensating process for each nozzle is varied according to the group into which a given nozzle is sorted.
According to another characteristic aspect of the present invention, an ink jet recording apparatus, that is, a recording apparatus which employs a recording head comprising a plurality of nozzles for ejecting ink, and which forms an image on recording medium by ejecting ink onto the recording medium, comprises: an image outputting means for outputting two or more images of a pattern, uniform in gradation, for measuring the recording properties of the recording head; a measuring means for measuring the density distribution of the outputted images of the pattern; a calculating means for calculating the data for compensating for each of the plurality of the nozzles, based on the results of the measurements of each image of the pattern; a sorting means for sorting the plurality of the nozzles into a plurality of groups different in properties, by making comparison among two or more sets of data corresponding to the aforementioned two or more images of the pattern; and a compensating means for making appropriate compensation for each group of nozzles, based on the properties which characterize the group, wherein the two or more images of the pattern outputted by the image outputting means are made different in the recording frequency at which the recording head is driven, and wherein the compensating means varies the compensation process for each nozzle according to the group into which the nozzle is sorted.
According to another characteristic aspect of the present invention, a method for compensating for the abnormal nozzles of the recording head of an ink jet recording apparatus, that is, a recording apparatus which employs a recording head comprising a plurality of nozzles for ejecting ink, and which forms an image on recording medium by ejecting ink onto the recording medium, comprises: an image outputting process for outputting images of two or more patterns, uniform in gradation, for measuring the recording properties of the recording head; a measuring process for measuring the density distribution of the outputted images of the two or more patterns; a calculating process for calculating the data for compensating for each of the plurality of the nozzles, based on the results of the measurements of each of the images of the two or more patterns; a sorting process for sorting the plurality of the nozzles into a plurality of groups different in properties, by making comparison among two or more sets of data corresponding to the aforementioned images of two or more patterns; and a compensating process for making appropriate compensation for each group of nozzles, based on the properties which characterize the group, wherein the two or more patterns, the images of which are outputted in the image outputting process, are different in the recording duty, and wherein the compensating process is varied according to the group into which a given nozzle is sorted.
According to another characteristic aspect of the present invention, an ink jet recording apparatus, that is, a recording apparatus which employs a recording head comprising a plurality of nozzles for ejecting ink, and which forms an image on recording medium by ejecting Ink onto the recording medium, comprising: an image outputting means for outputting images of two or more patterns, uniform in gradation, for measuring the recording properties of the recording head; a measuring means for measuring the density distribution of the outputted images of two or more patterns; a calculating means for calculating the data for compensating for each of the plurality of the nozzles, based on the results of the measurements of each of the images of the two or more patterns; a sorting means for sorting the plurality of the nozzles into a plurality of groups different in properties, by making comparison among two or more sets of data corresponding to the aforementioned images of the two or more patterns; and a compensating means for making appropriate compensation for each group of nozzles, based on the properties which characterize the group, wherein the two or more patterns, the images of which are outputted by the image outputting means, are made different in the recording duty, and wherein the compensating means varies the compensation process for each nozzle according to the group into which the nozzle is sorted.
The recording duty of one of the two or more shading patterns is no more than 50%, and that of another is no less than 50%. The recording duty is desired to be set in consideration of the balance between the recording duty value and gradation value. Further, three or four head shading patterns different in recording duty (for example, 25%, 50%, 75% and 100%) may be employed. The employment of a larger number of shading patterns different in recording frequency, recording duty, or the like, makes it possible to create more accurate data regarding the refilling properties of the nozzles of a recording head.